Heat only turns on after cycling the engine?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Stefenn Fiscus, Nov 13, 2021.

  1. Stefenn Fiscus

    Stefenn Fiscus New Member

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    2009 with 180K. I've been searching far and wide but can't seem to find anyone with this particular odd ball issue. Everything on the car works great, I keep the maintenance up and have basically replaced everything when or before it breaks. Love this car. I'm trying to figure out what is going on with the heater so I can fix that too. When you turn the car on and drive it with the heat on full blast.. no heat. Just blows cold/room temp air. When you turn the car off, and turn it back on.. suddenly the hot air comes back and works just fine. On a few occasions cycling it twice was needed but more often than not just once works.

    Anyone have any idea? I'm not afraid to tackle anything on this car but would love to know where to start before I just start throwing money at a heater core, blower motor, thermostat, etc.
     
  2. Mendel Leisk

    Mendel Leisk Sand Pounder

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    If you continue to drive without shutdown, it never gets hot? I know if you have the system is on auto, it won't start blowing till the engine coolant has warmed sufficient to make it worthwhile.
     
  3. sam spade 2

    sam spade 2 Senior Member

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    Critical question.
    But I assume that he knows that the engine must run for a few minutes before he should expect any heat.

    My guess is a sticking blend door, not routing the air through the heater core properly.
    But verifying and fixing that might be a complicated procedure, involving taking the dashboard apart.

    Simply turning on the front defroster and then turning it back off again might get the air flow back to heat mode.
     
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  4. ChapmanF

    ChapmanF Senior Member

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    In a Gen 2, there are a couple things that can affect the temperature of the air from the heater.

    The blend door, mentioned by Sam, controls whether the air goes through the heater core, or around it.

    The Gen 2 coolant control valve, under the hood, controls whether the hot coolant goes through the heater core or around it.

    Clearly if either one isn't going through the core, you don't get heat. And the blend actuator and the coolant control valve actuator are things in Gen 2 that can sometimes become flaky and intermittent.

    Do you have any warning lights on the dash?

    Blend actuator problems might not light any dash lights, but may set HVAC codes that you can easily retrieve by holding AUTO plus FRESH/RECIRC while powering the car on.

    A coolant control valve issue is likely to set an engine DTC (which a scan tool can retrieve) and light the check-engine light.
     
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  5. Stefenn Fiscus

    Stefenn Fiscus New Member

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    Wow, did not expect so many replies so quick, thanks everyone.

    A few things:

    1) Yep, the car is running for what I would say is a sufficient amount of time. Ive owned the car for about 5 years so I know generally how long it takes to heat up. That's actually how I found the issue in the first place, after it was running for a while I thought "Huh.. usually we have heat by now." The issue has happened sporadically since last year but seems to be worsening to every trip now.

    2) One of the upsides of being frugal and loving to wrench is I've done all the maintenance on this car including buying an OEM Hybrid Battery from Toyota and swapping out the old once I got the Triangle of Death. One of the bad sides is I'm usually less than great about tracking what I've done and I've had the car for so long. I'm almost positive I'm positive I changed out some small part on the front of the engine related to the coolant right when I first bought it. Fairly confident now that I'm reading it above it I was the coolant valve.

    3) No codes. I bought it with the air pressure light code on due to those damned TPC sensors and once I fixed that I'm proud to say no codes since. That is until I had to get put in a new Hybrid Battery

    4) Blend Door Actuator: This is where I was going to start first, but given it isn't making any audible noises I figured I'd check the forums first. The heat blows hot when it does kick on which leads me to believe it is a routing issue. The blend door is cheap/easy enough to fix and always start with the simplest answer first so that may be the route.

    I admittedly don't know a ton about heater cores. My assumption would be that when a heater core is failing it would ween off slowly over time until essentially you have no warm air - rather than snapping from hot to cold like I'm experiencing. Is that accurate?

    What codes would you see for the FRESH/RECIRC? What is the output/unit of measure I'd be looking for?

    Thanks all!
     
  6. mr_guy_mann

    mr_guy_mann Senior Member

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    It would be quite useful to have a capable scantool like Toyota Techstream to view data from the HVAC system, such as blend door position. Check if any data changes after cycling the key, any codes are set, etc. Also might want to see if you can reach a hand to carefully feel both of the heater hoses (while you have the cold air fault) and see if both are hot with good coolant flow (or not).
     
    #6 mr_guy_mann, Nov 15, 2021
    Last edited: Nov 15, 2021
  7. ChapmanF

    ChapmanF Senior Member

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    When you hold AUTO + FRESH/RECIRC and power the car on, you will see a test pattern shown on the MFD and then a more-or-less normal display with two-digit diagnostic codes in the top left. They are shown in units of "look this code up in the repair manual". :)
     
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